The present invention relates generally to tracking control apparatus for reproducing recorded signals from rotary recording mediums. More particularly, the invention utilizes a main information signal and pilot signals for controlling tracking by the reproducing transducer which moves accurately and stably over the main information signal track of the recording medium.
Known systems record and reproduce information signals (such as video signals and/or audio signals) on and from disc-shaped, rotary mediums (referred to hereinafter simply as discs). These are broadly divided into systems using optical reproduction, systems using a reproducing stylus employing a piezoelectric element, and systems utilizing variations in the electrostatic capacitance between an electrode on the reproducing stylus and the recorded surface of the disc.
The advantages and disadvantages of these three kinds of systems have been considered. It is concluded that the electrostatic capacitance system is most desirable. Further, the present applicant has previously described, as a new system for overcoming the problems accompanying known electrostatic capacitance system, a novel "Information signal recording system" as disclosed in the specification of U.S. patent application Ser. No. 785,095, filed Apr. 6, 1977.
More specifically, in accordance with a known electrostatic capacitance system, recording is accomplished by forming a spiral guide groove in the disc for guiding the reproducing stylus. At the same time, pits are formed in responsive correspondence with an information signal to be recorded on the bottom surface of the groove. In the reproducing system, the reproducing stylus is guided by the guide groove. The stylus traces the track within the groove and reproduces the signal in response to variations in the electrostatic capacitance.
In this known system, however, a reproducing stylus guiding groove is provided in the disc. The reproducing stylus is compulsorily guided by this guide groove. It is not possible for the reproducing stylus to undergo operations such as riding over the groove wall of one track, moving into another track and returning to the original track. If the reproducing stylus were to be forced to undergo such an operation, the groove and the stylus would be damaged. For this reason, it has not been possible in this known system to carry out special reproduction such as quick-motion picture reproduction, slow-motion picture reproduction, and still-picture reproduction.
Furthermore, since the area of contact between the tip of the reproducing stylus and the guide groove of the disc is small, the stylus easily wears. When the width of the stylus tip reaches the groove width as a result of abrasive wear, the serviceable life of this reproducing stylus ends. Thus, this system is accompanied by the problem of short life of the reproducing stylus. Another problem is that fine chip particles abrade from the disc by the tracing action of the reproducing stylus along the guide groove. These chips give rise to difficulties which promote the wear and damage of the stylus and the guide groove. This wear increases the gap between the electrode at the stylus tip and the recorded surface including the pits within the disc guide groove. The reproduced output decreases because of this spacing loss.
If the rotational speed of the disc is set at a low value such as 450 rpm., for example, it will prolong the life of the stylus tip. However, wavelength of the information signal recorded in the guide groove will become short. For this reason, a low price recording apparatus cannot employ laser light in the recording system. The disadvantageous high price of a recording apparatus using an electron beam becomes unavoidable.
Accordingly, an aim of the invention is to overcome the various problems of the known systems described above. The present inventor has described in the aforecited patent application a novel system for recording and reproducing information signals. According to this proposed system, the recording system forms pits in accordance with the information signal being recorded along a spiral track on a flat disc shaped recording medium, without forming a groove therein. In the reproducing system, a reproducing stylus traces over and along this track to reproduce the recorded information signal. In this system, pilot or reference signals are recorded on or in the vicinity of a track of the information signal, such as a video signal, on a rotary disc. At the time of reproducing, the reference signals are reproduced together with the video signal. Tracking servo control is carried out so that the reproducing stylus traces accurately along the track in response to the reproduced reference signals.
By the use of this previously described system, the recording track has no groove. There is no possibility whatsoever of the reproducing stylus or the recording medium being damaged. The stylus can trace the same portion of the track repeatedly many times, whereby a special reproduction such as still, slow motion, or quick motion reproduction becomes possible. Furthermore, other difficulties of the known system are removed.
Furthermore, a reproducing stylus of highly desirable characteristics may be used for reproducing an information signal recorded on a disc without a reproducing stylus guide groove. The present inventor has previously described a novel "Stylus for reproducing information signals recorded on a recording medium" as disclosed in the specification of U.S. patent application Ser. No. 852,366. This proposed reproducing stylus has a bottom surface with a maximum width portion which is greater than the track pitch and has an electrode of a width smaller than the track pitch over a specific distance from the stylus tip.
A feature of this proposed reproducing stylus is that the stylus pressure per unit area is small. There is a low rate of abrasive wear of the reproducing stylus and the disc. Another feature of this stylus is that, the electrode width does not increase appreciably even when the stylus tip becomes somewhat worn after a long period of use. The life of the reproducing stylus is remarkably long.
Thus, by using the recording and reproducing system and the reproducing stylus according to the above cited applications, the problems encountered in the prior art are overcome. Very good results can be obtained.
In the above mentioned recording and reproducing systems, two pilot signals of mutually different frequencies are recorded on the opposite sides of each track carrying the main information signal. When the tracking of the reproducing stylus deviates relative to the main information signal track of the recording medium, the reproducing stylus reproduces a pilot signal together with the main information signal. The tracking control circuit operates in response to the pilot signal to produce an output tracking error signal corresponding to the direction and magnitude of the tracking deviation. The reproducing stylus is controlled in response to this error signal so as to track accurately over and along the main information signal track.
However, the level of the reproduced signal varies with the radial positional displacement of the reproducing stylus on the rotary recording medium. The reproduced level also depends upon variation in the contact between the electrode of the reproducing stylus and the surface of the rotary recording medium, dust and grime adhering to the surface of the rotary recording medium, and undesirable abrasive wear of the electrode part of the reproducing stylus.
Where there is a fluctuation of this nature in the level of the reproduced signal, the level of the reproduced pilot signal also fluctuates. However, a DC error voltage is obtained in response to the fluctuating level of the reproduced pilot signal. This error voltage is applied to the mechanism for controlling the tracking stylus, but the loop gain of the control circuit always varies together with variation of the level of the reproduced signal. In this case, the control loop is disrupted, and stable tracking control is not attainable, whereby erroneous tracking may occur.